The general objective of the research is to define the long range structure of DNA in isolated chromosomes and to delineate the molecular interactions which organize this structure. Bacterial chromosomes and condensed chromatin from Drosophila will be studied. In both cases methods are now available for isolating these structures free (within detection limits) of double-strand breaks in the DNA. Physical chemical methods using ultracentrifugation techniques have been developed to study changes in the folded state of the DNA and to quantitate the segregation of the DNA into separate domains of supercoiling. Quantitative electron microscopy and autoradiography procedures will also be used to measure changes in the organization of the packaged DNA. The experimental approach is essentially a molecular dissection of the chromosomes. The dissociation of certain components of the chromosome or chromatin is to be correlated with changes in the organization of the DNA. Conversely, reconstitution of purified protein components and the partially unfolded chromosome will be investigated to define specific roles of the chromosomal proteins.